The present invention relates to a technique for use with a radio frequency IC tag, and in particular, to a technique of matching impedance for a micro-strip antenna to be mounted on a radio frequency IC tag.
A radio frequency IC tag is capable of communicating information by radio, for example, transmitting therefrom information such as an IDentification (ID) number stored in the IC tag. Hence, a reader/writer device which communicates with the radio frequency IC tag can conduct a contactless operation to read the information recorded in the IC tag without making contact with the IC tag. Thanks to the radio communication, the information recorded in the IC tag can be read therefrom even if the IC tag is placed in a bag or a box. Therefore, the radio frequency IC tag is broadly used for production management and distribution management of articles.
The radio frequency IC tag includes an IC chip having recorded information and an antenna to communicate by radio the information recorded in the IC chip. Various types of antennas are available for the IC tag. A representative example is a dipole antenna in which the terminals of the IC chip are respectively connected to peripheral ends of two metallic plates. Due to the simple structure and the low unit price, the dipole antenna is suitably employed when it is attached onto a large number of articles. However, when the article onto which the radio frequency IC tag is attached is made of a metallic material or a material containing moisture, e.g., a meat, a living body, or a vegetable, the communicable distance of the IC tag rapidly drops and the communication is disabled depending on cases. However, as commonly known, a micro-strip antenna is capable of securing a stable communicable distance even if the radio frequency IC tag is attached onto the articles described above.
In general, the micro-strip antenna includes a radiation electrode, a ground conductor, and a dielectric interpolated between two conductors, i.e., the electrode and the conductor. The antenna is powered by connecting the radiation electrode to the ground conductor. When the micro-strip antenna is employed in a radio frequency IC tag, both terminals of the IC chip mounted in the IC tag are connected to the power feed points of the antenna.
However, in the micro-strip antenna, the conductors are connected through the dielectric by use of the IC chip terminals. When the antenna is pressed by external force and is deformed, the distance between the associated components of the antenna changes and hence the connection is disturbed.
Description will now be given of impedance matching between the IC chip and the antenna.
Impedance of the IC chip includes a resistance component and a reactance component. This is also the case with impedance of the antenna. For example, if the reactance of the IC chip is the capacitance component and the reactance component of the antenna is the inductance component, influences from the respective components can be mutually cancelled out. Hence, a current obtained by the antenna is efficiently fed to the IC chip in operation. However, if the IC chip is connected to the antenna with discrepancy between the capacitance component and the reactance component, namely, with impedance mismatching, it is not possible to efficiently feed the current from the antenna to the IC chip. This leads to reduction in the communicable distance of the radio frequency IC tag. The known techniques to establish impedance matching in this situation include the technique to change the power feed position of the antenna, the technique to connect a coil and a capacitor to the antenna, and the technique to provide structure called “slit” in the power feed section (JP-A-2002-135029).